The present invention relates to a method of analysis of signals, notably vibrational signals, generated by the rotation of a drill bit.
In the drilling technique, be it intended for oilfield development or for other purposes, a drill bit screwed on to tubes whose assembly is commonly called drill string is used. The drill string is the mechanical link between the bottom of the hole drilled and the ground surface. The drill rig is the surface installation which notably drives the drill bit into rotation, assembles and bears the weight of the drill string, injects a fluid into the drill string. In a variant, the bit can be driven into rotation by a downhole motor assembled in the drill string. A compression stress called weight on bit (WOB) has to be applied on the bit so that the bit can destroy the rock. To that effect, drill collars are assembled above the bit. Drillpipes complete the drill string.
The drill man who drives the drill rig knows some parameters which condition the action of the drill bit: the weight hanging on the pipe hook, the rotary speed and the torque applied by the rotary table, the flow rate and the pressure of the fluid injected. These parameters, measured at the surface, are used for running the drilling operation without knowing the real working conditions of the bit at the bottom of the well.
Downhole devices for performing recordings and transmissions of measurements provided by downhole sensors have been developped. These devices, commonly called MWD (Measurement While Drilling), are mainly intended for transmitting towards the ground surface the geographic position of the drill bit. However, they can also include load sensors that measure the stresses in the drill collars located above the bit.
Document EP-A-0,558,379 also describes a measurement system located in the drill string, close to the bit, the system being connected to the surface by at least one electric conductor.
There are thus well-known means allowing acquisition of data relative to the dynamic behaviour of the drill string during the drilling operation.
However, in order to run the drilling in an optimized manner, having signals representative of the behaviours of the drill bit is not sufficient, these signals also have to be interpreted in order to know the drilling process. In fact, the vibrational signals generated by the drill bit are complex signals which can provide a good representation of the evolution of the drilling operation, an operation which is not linear but which is a succession of different phases and behaviours.
A first problem encountered in the presence of such signals is the detection of one behaviour among the others. When these behaviours have been detected, the characterization of each behaviour can then be contemplated. Once characterization is completed, it will allow given behaviours to be recognized and predicted, and only characteristic coefficients to be processed or transmitted.
In certain signal instances, one may merely segment the signal into segments of a duration determined a priori, then each segment can be characterized according to the method of the invention. Once characterization is completed, analyses, processings or transmissions of the characterization coefficients can be performed. Furthermore, the means used can allow the original signal to be reconstructed from the characterization coefficients.
Optimization can consist in interpreting the vibration ranges of the drill string, notably at the level of the drill collars, in order to detect certain dysfunctionings of the drill bit. Certain dysfunctionings are well-known, such as stick-slip, where the rotation of the bit is very irregular until the bit jams, bit bouncing where the bit comes off the working face, or whirling where the bit moves in an uncoordinated precession motion. Analysis of the vibrations due to the reaction of the bit on the rock can also allow the changes of nature of the rocks to be detected, and maybe even the specific nature of a rock, the wear of the bit edges or the balling up of the bit (bad cleaning of the edges) to be identified.
To that effect, a method of analysis of the signals representative of the vibration ranges generated by the running of the bit is necessary.
One of the methods used is based on the use of the Fourier transform. The signal is decomposed into an infinite amount of sinusoids. However, when non stationary phenomena such as the behaviours of a bit during drilling are to be studied, the Fourier transform is inadequate. In fact, it appears that the most pertinent information is to be found in the non stationary parts of the signal.